1) antenna size = approx wavelength of frequency of transmission
eg. if freq of TX=1KHz => Lamba= wavelength=300000meters!!!!!
this size is impractical..thus we need to use higher frequencies
2) Suppose we want to TX music from A to B...the frequeny range of
music is generally from 20Hz to 20000Hz then the sizr of the antenn
required will range approximately from 15000Kilometers to
15kilometers for TXing the different frequencies.
firstly it is quite
obvious that it is impossible to firstly build an antenna of these sizes at
all!!! and even if you build it how can you use one single antenna to TX
the entire range which will have an infinite number of frequencies.
Which is why we use MODULATION btw....if we modulate using a
simple DSB-AM...around carrier at say 50MHz....then the entire
frequency range we need to transmist is only from 49.98MHz to
50.02MHz...so then we only need antenna sizes of the order of
6.0024meters to 5.9976meters...these sizes are much more practical
to achieve and since the range of antennas needed is almost constant
relative to an unmodulated TX we might as well use a single antenna
size with a negligible loss in TXed power.
3) Also...in general the atmosphere or air or whatever allows
transmission of EM waves at a higher frequency with much less
attenuation than with low frequencies...this is just a property of the
atmosphere and since we cannot do anything about it...we must use a
high frequency of TXission.
There are a lot more reasons which are more complicated to explain.